How to identify scammers
Here is a small list of signs or hints that can let you believe that the person who is selling a dog is a scammer
 

1. First of all, the low price. Most scammers attract their victims by selling puppies for just a couple of hundred dollars with or without shipping included. Sometimes they will give it for free and ask you $400 or $500 for shipping costs.

2. The type of payment. A scammer (or a robber) would never accept credit cards simply because credit cards and the merchant (which is service needed to process credit cards) are bank institutions and they will protect the buyer if the seller is running scams. Most will take wire transfers, money orders or cash on an envelope only. To protect yourself, ask to pay with credit cards, paypal or escrow.

3. The origin of the seller. Despite some scammers are in United states, most are posting their classifieds for free puppies, etc. from other countries.

4. Lack of Reputation. If you are not paying with a secure method like credit cards, Paypal or escrow, at least check their references. Honest sellers rely on their references to show their reliability, credibility and gain trust on new prospective buyers, and they would never try to sacrifice their reputation scamming a buyer.